1. Field of the Invention
The present invention relates to a method of making a half-tone gravure plate, and more particularly to a method of making a gravure printing plate in which a planographic or relief half-tone positive and a gravure screen are printed on a photosensitive resist forming material.
2. Description of the Prior Art
In the past, a method is well known and extensively embodied as can be seen in Japanese Patent Application Laid-open No. 50-26603 which comprises printing a planographic or relief half-tone positive and a gravure screen on a photosensitive resist forming material, thereafter subjecting said photosensitive resist forming material to development and other processes to form a resist, and etching a gravure printing plate material through said resist to obtain a gravure printing plate.
In such a method, a moire pattern is sometimes formed depending on a combination of a screen angle of the half-tone positive and a screen angle of the gravure screen to lower the quality of images. Accordingly, in such a case, it is necessary to combine the screen angles of the half-tone positive and gravure screen with an angle to make the moire imperceptible in order that the moire pattern is made imperceptible. Such an angle may be obtained by the relative rotation of the half-tone positive or the gravure screen.
On the other hand, in the step of making a gravure printing plate, positive designs for several pages (for example, four pages) are often formed on a single cylinder at the same time, in which case, a sheet of large gravure screen is printed in contact with a sheet of large photosensitive resist forming material, and a half-tone positive for several pages is printed on the same photosensitive resist forming material to prepare a resist. In such a case, if the screen angles of all half-tone positives are the same, no problem arises but if a screen angle of one half-tone positive for any of pages is different from that of the other half-tone positive, a problem arises.
That is, if the screen angle of the half-tone positive for any of pages is made to minimize the moire pattern, other half-tone positives do include one which has a screen angle at which the moire pattern is formed. Such a problem is not only involved in the case that the screen angle is different at page unit but in the case that a design having different screen angles is present even in one page.
Furthermore, if the number of screen lines (screen ruling) of the half-tone positive is different from the number of screen lines (screen ruling) of the gravure screen, the angle at which the moire is minimized is different.
Thus, in the case that the number of screen lines of the half-tone positive is different at page unit, or also in the case that the number of screen lines of any design in the half-tone positive for one page portion is different, there occurs a problem in the formation of moire pattern between the half-tone positive and the gravure screen similarly to the foregoing.
Also, it is sometimes necessary to print a design previously printed by the planographic printing system, by the gravure printing system at a different magnification. In this case, the half-tone positive used in the planographic printing can be enlarged or reduced as it is to make a half-tone positive having the desired magnification, but if the magnification of enlargement is different with the page unit or design, the number of screen lines of the half-tone positive becomes varied according to the magnification of enlargement, which leads to the problem of formation of the moire pattern between the half-tone positive and the gravure screen as previously described.
Accordingly, if any inconvenience occurs because of irregularities in the screen angle, the number of screen lines, the magnification of enlargement, etc. of the half-tone positive as described above, it is apparent that all that need be done is to arrange them so that they may have the same value.
Incidentally, a method is disclosed in Japanese Patent Application Laid-open No. 55-28044 which can vary at least one of the screen angle, the number of screen lines, the shape of dots and the enlargement ratio of the half-tone positive to prepare another half-tone positive. The aforesaid patent application discloses a method in which a half-tone positive as an original is exposed to a photosensitive material having a contact screen held in contact therewith at the desired magnification with the diaphragm value of a lens of a plate making camera set to more than 64, preferably, from 64 to 128, to thereby prepare a new half-tone positive having the desired screen angle, the screen ruling, shape of dots and enlargement ratio. According to the method disclosed in the aforesaid patent application, in the case that the conversion is accomplished between the half-tone positives at an angle at which the moire is relatively imperceptible so as to assume 30 degrees which is a difference in the screen angle between the original half-tone positive and the new half-tone positive, the moire is imperceptible even if the dots of the original half-tone positive remain without being completely eliminated, and therefore, this method is effective to some extent.
However, in case of the present invention, it is necessary to convert the half-tone positive for some pages out of the half-tone positive for several pages into the screen angle of the half-tone positive for other pages or to convert some designs in one page into the screen angle of other designs, in which case, the difference in screen angle between the original half-tone positive and the new half-tone positive is not always 30.degree. at which the moire pattern is imperceptible but is sometimes 5.degree. or 10.degree..
The present inventor has found that in such a case as described above, when the diaphragm value of the lens as disclosed in the aforementioned Japanese Patent Application Laid-Open No. 55-28044 is set from about 64 to about 128, the moire pattern is still perceptible to lower the quality of printed matter. As the result, the present inventor provides a new method of making a gravure printing plate, which method comprises preparing a new half-tone positive by a totally separate method and using said new half-tone positive to obtain a gravure printing plate whereby the lowering of the quality of printed matter resulting from the formation of moire pattern can be prevented and an error of graduation caused by the reproduction step between the new half-tone positive and the original half-tone positive can be corrected.